The present invention relates to circuit making and breaking devices, and particularly to their contact structures.
The present invention will be considered in connection with "circuit breakers" although it will become apparent that the novel features are widely applicable to other forms of circuit making and breaking devices. The term "circuit breaker" as used in the art signifies apparatus for making and breaking a circuit at any level of current up to its rated current and extending to over-currents and into the short-circuit range. Circuit breakers having rated currents of several hundred to several thousand amperes may be used in circuits where short-circuit current may reach 75,000 amperes for example. Circuit breakers are required to interrupt short-circuits repeatedly, safely and non-destructively. They are also required to close safely and non-destructively where there is a pre-existing short circuit at the load side of the circuit breaker, resisting electro-dynamic forces that might damage or even weld the contacts if there were any hesitation about completing the contact-closing operation. After closing on a pre-existing short-circuit, the circuit breaker mechanism is of course activated promptly into its opening operation, for safely and effectively interrupting the flow of current.
Circuit breakers for use in the 480 volt and 600 volt class are of the so-called "air-break" class, where an arc that develops in the course of current interruption is quenched in air in an arc chute, without benefit of coils on magnetic cores to drive the arc away from the contacts and into the arc chute. Large circuit breakers almost always have so-called main contacts and arcing contacts. The closing operation involves initial engagement of the arcing contacts which momentarily carry the current of the circuit. The main contacts close immediately afterward and virtually all of the current is then diverted away from the arcing contacts. During a reverse operation for interrupting current flow, the main contacts part initially, transfering the current to the arcing contacts, and when these contacts part, an arc develops which expands into the arc chute where it is quenched. Arcing should not occur at the main contacts when they part and, consequently, the main contacts remain clean, and provide a low contact-resistance current path through the circuit breaker while it is closed. This is true even during the very short time when short-circuit current flows and until the main contacts part and cause transfer of the current to the arcing contacts.
Traditionally, circuit breakers designed for meeting these arduous conditions have utilized so-called "butt" main contacts and either butt or "knife-blade" arcing contacts. When butt contacts close, the high speed of the moving contact tends to cause contact "bounce", which in turn may cause slight contact-damaging arcs. Powerful closing-spring mechanisms are usually provided to minimize contact bounce.
So-called "knife-blade" contacts have been used in switches designed for heavy currents, but "knife-blade" contacts have rarely been used as the main contacts of circuit breakers. Some finite time is spent while a moving knife-blade contact parts from companion stationary contacts in a wiping-contact motion; and under these conditions, during opening operations, small but damaging high-current arcs may develop at times, with resulting arcing damage to the contact surfaces. Thereafter, when the arc-damaged contact surfaces are again in engagement, objectionably high contact resistance and excessive heating tend to develop at the contacts. Worse, still, is the possibility of knife-blade contacts becoming welded together. Butt contacts tend to act abruptly in parting operations, and minimize these problems.
Knife-blade contacts are commonly used in simple switches, which are not intended to open under short-circuit conditions. Knife-blade contacts provide broad contact areas and they tend to be self-polishing due to the wiping action in the opening and closing motions of the switch, representing a low-resistance cool-operating contact configuration. Low resistance depends in part on the contact pressure; and in order to achieve high-contact pressure, powerful switch-closing effort must be available. An adaptation of the knife switch is the so-called bolted-pressure switch. These are basically knife-blade switches wherein, during the final operation of the closing mechanism, the fixed and moving contacts are tightened against each other by a threaded or cammed clamping mechanism, activated by the arm that carries the moving contact or by its operating means. The clamping mechanism is activated near or at the fully-closed phase, so that the requisite closing effort is minimized even though high contact pressure is developed.